The present invention relates to a Yankee dryer made of steel for drying a pulp web, with a cylindrical steel shell that is connected to a first and second cylinder end cover at either end, where the cylinder end covers each have an opening at their centre, and to a process for manufacturing a Yankee dryer.
In production of paper webs, particularly for tissue, it is common practice to use so-called Yankee dryers in the drying process. Yankee dryers usually have a very large diameter. They are heated with steam and are difficult to manufacture because there are high demands to be met concerning internal pressures, leak tightness, and the large diameters.
Yankee dryers customary in the trade have the following dimensions, for example:
Cylinder diameter:3000 mmto 5500 mmDiameter of Hollow shaft:1500 mmto 1800 mmCylinder width:6000 mmto 7500 mmCylinder mass:40 tto 95 t
These cylinders are made predominantly of cast iron, however Yankee dryers made of steel are also already known from U.S. Pat. No. 4,196,689 and DE 2707923. Normally a Yankee dryer consists of a cylindrical shell surface that is closed off at the ends by cylinder end covers of various shapes. The two covers can be bolted or welded to the cylinder shell.
A Yankee dryer rotates in bearings by means of journals and contains a hollow shaft or axle through which the steam for heating can be fed to the cylinder and the waste steam and condensate can be removed.
WO2008/105005 discloses a Yankee dryer made of steel with a central shaft in several sections. This means that the two bearing journals of the Yankee dryer are attached to the end covers separately from a hollow shaft inside the dryer, using bolts for example (see FIG. 1). This embodiment has the disadvantage that a large number of bolts and flanges are required, which weaken the component. Furthermore there are more components with tolerances to be machined, thus there may be imprecise bearing alignment.
Conventional Yankee dryers, as illustrated in WO2008/105005, are manufactured in the following assembly sequence:                1. The cylinder shell is placed on the first end cover and joined to it (bolted or welded).        2. The hollow shaft with the steam and condensate pipes attached to it is placed inside the cylinder.        3. The hollow shaft is welded or bolted to the first end cover.        4. The second end cover is placed on the cylinder shell and joined to the cylinder shell and the hollow shaft.        5. The bearing journals are affixed to the end covers.        
Due to the safety regulations for manufacture of pressure vessels, the end covers must be welded to the shell on both sides, i.e., on the inside and on the outside. The conventional assembly sequence has the disadvantage that the root weld between the second cover and the cylinder shell on the inside of the cylinder can only be made by entering the cylinder through the manhole in the end cover. This leads to more difficult working conditions for the welders on the one hand (lack of air, light, and space) and presents a much higher risk of accidents on the other hand.